This invention relates to metal tubing which has the efficiency of heat transfer of its surface enhanced. It is known in the art that modifying the surface of a plain cylindrical tube such as by finning or corrugating it or by scoring, knurling, or roughening its surface will increase its heat transfer capability in boiling of liquids substantially as compared to a plain tube. U.S. Pat. No. 3,454,081 teaches beneficiation of a planar heat transfer surface via a scoring and knurling technique in which the ridges formed by scoring are partly deformed, by a subsequent knurling operation, into the grooves separating them to obtain partially enclosed and connected subsurface cavities for vapor entrapment and the consequential promotion of nucleate boiling. U.S. Pat. Nos. 3,326,283 and 3,602,027 teach knurling after finning while U.S. Pat. Nos. 3,683,656 and 3,696,861 teach partially bending over the fins to form cavities. U.S. Pat. No. 3,789,915 teaches that a tube may be corrugated or knurled (but not both) to get a folded metal configuration having subsurface cavities. U.S. Pat. No. 3,355,788 relates to heat transfer of gas rather than boiling liquids and discloses the use of saws to slit finned tubing to increase turbulence. As a general rule, finning has always been done on plain smooth surfaces because it is known that surface imperfections can lead to fin splits of uncontrolled depth and orientation. This type of fin imperfection has been avoided in the past for fear that splits penetrating into the tube wall might cause undue degradation of mechanical properties. Splits are now identified non-destructively by eddy current test methods, which are applied at the tube manufacturing stage. Generally, splits are not permitted to penetrate deeper than 10% of the tube wall thickness-- a tube having excessively deep splits is scrapped. However, splits in the fin tips and those not penetrating deeper than 10% of the tube wall thickness are acceptable.
Since heat transfer tubing is generally made of expensive materials such as copper and is used in large quantities, it is obvious that improvements in heat transfer efficiency and/or in manufacturing costs can be quite significant in reducing the overall cost of a given heat exchange installation.
It is an object of this invention to provide finned tubing which has increased heat transfer capability compared to conventional finned tubing in nucleate boiling applications.